In graphic arts technology there is a need for reproduction of continuous tone images by an imaging device. The imaging device, for example, a laser based imager is capable of producing a limited number of levels representing optical parameter (in most common cases only two levels are used—black and white). This goal is achieved by applying a process called screening, wherein a gray tone value which represents a pixel to be screened in the original image is simulated by means of varying relative area covered by dark elements (pixels) as compared to light elements.
Different screening methods exist. In one of the most common, an area of an image is reproduced by subdividing it into equal, periodically repeated sub-areas called mesh cells, containing variable-size dark elements (printing dots, alternatively called halftone dots). Relative area coverage is defined as ratio of dark element area to a mesh cell area. Such screening method is commonly called Amplitude Modulation (AM) screening. Such screen, with a regular, usually square grid structure, is characterized by a screen period and a screen angle. The reciprocal of this period is called screen frequency or screen ruling.
Particular problems arise when reproducing characteristics of printing dots which are size dependant. Examples of such processes are flexographic, offset and xerographic printing processes.
In flexographic printing, the size of the smallest halftone printing dot on a printing plate that can be consistently reproduced on press is usually around 40 microns in size. Below this size, halftone dots tend to print unevenly, and may also include drastic increase in size and produce large blot artifacts, or not printing at all. For commonly used line ruling of 120 lpi and commonly used device resolutions of 2400 or 2540 dpi, 40 micron halftone dot corresponds to area coverage of 3%; for line ruling 1500 lpi, it corresponds to area coverage around 4%. This may result in discontinuity, with annoying artifacts in certain cases especially in the highlight parts of printed images. The high dot gain that is associated with flexographic printing process enhances the effect and exacerbates the problem.
In the offset printing process, the minimal halftone printing dot that can be reproduced consistently can often be as small as 10 micron. For lower quality paper and high-speed printing presses, for example, in newspaper printing, the same fundamental problem discussed above exists. A similar situation exists in electro-photographic printing, wherein minimal printing dot size is often defined by physical characteristics of toner particles.
One solution for the above problems is using Frequency Modulation (FM) screening techniques with controlled minimal dot size, so-called “green noise” frequency modulation, shown in U.S. Pat. No. 5,689,623 (Pinard), or with controlled midtone clustering, so-called “second order” frequency modulation, shown in U.S. Pat. No. 5,579,457 (Hall). While solving the problem of highlight region reproduction, frequency modulation introduces its own drawbacks. Relatively rough feature size needed for proper highlights reproduction, often leads to grainy appearance both in highlights and in the midtones areas. Additionally, high circumference-to-area ratio inherent for FM generated printing dots leads to significantly higher dot gain compared to AM halftone screening. Considering that flexographic printing process is already characterized by high dot gain, FM screening may lead to significant contraction of the dynamic range for printed images.
Another solution is known as the “double dot” technique or “Respi screen.” According to this technique, the extremes of tone scale, highlights and/or shadows, are rendered with halftone dots that are laid on a grid with the same angle, but with the frequency of the square root of the halftones in the rest of tone scale, thus halving the number of halftone dot and, consequently, doubling the size of each dot. This renders the transition area between extreme and main part of tone scale with halftone dots of two different sizes placed in checkerboard pattern. While moving the cutoff value for non reproducible part of the image farther to extreme parts of tone scale, it still does not completely solve the problem. Moreover, by introducing additional screen frequencies, such technique may produce highly undesirable moiré effects in multi-colored images in case of regions where part of color separations are in extreme parts of the tone scale and other separations are in non-extreme part of the color scale.
In order to overcome the deficiencies above stated, an approach was proposed by U.S. Pat. No. 5,766,807 (Delabastita et al.). This method is known as “hybrid screening.” In this approach, a “supercell” threshold matrix suitable for periodically tiling a plane is defined in such a way that it contains a plurality of locations for halftone dot centers and is filled with threshold values. When the matrix is used for screening a contone image, in the extreme parts of the tone scale halftone dot of predefined minimal size are produced, whereas in the remaining part of halftone dot centers no halftone dot are produced at all. This is performed in such a way that the area coverage for a whole supercell area corresponds to a tone value in the contone image. In other words, instead of modulating halftone dot size, below predefined dot percentage, dot size is kept constant but dot number is modulated as a function of tone value; accuracy of tone representation being defined by predefined minimal halftone dot size and count of halftone dot centers in supercell threshold matrix.
While free of most undesirable effects of previous solutions, this approach still has some problems. Notably, the supercell-based pattern is prone to grainy and “noisy” appearance; relatively rough quantization steps limited by number of halftone dot centers in a supercell threshold matrix may produce banding effects in vignette parts of image; “orphaned” and incomplete halftone dots still may produce undesirable “blot-like” artifacts; and supercell-based approach limits available number of screen angle/screen frequency combinations to those with rational tangent angles.